net/mlx5: Remove unused 'offset' field from mlx5_sq_bfreg

net/mlx5: Remove unused 'offset' field from mlx5_sq_bfreg

The 'offset' field was introduced in the original commit [1] and never
used until commit [2], which added an unnecessary use.

Remove the field and refactor the write-combining test to use a local
variable instead.

[1] commit a6d51b68611e ("net/mlx5: Introduce blue flame register
allocator")
[2] commit d98995b4bf98 ("net/mlx5: Reimplement write combining test")
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>